Field of the Invention
The invention relates to a circuit configuration containing memory devices, which contain programmable elements and volatile memory elements, for monitoring the states thereof.
For repairing faulty memory cells, integrated circuits, particularly integrated memories, have redundant word lines or redundant bit lines, which can replace regular lines containing faulty memory cells on an address basis. In this case, the integrated memory is, by way of example, tested by a self-test device, and the redundant elements are subsequently programmed. A redundancy circuit then has programmable elements, for example in the form of electrical fuses, which are used for storing the address of a line that is to be replaced. The electrical fuses are electrical connection elements whose line resistance can be varied, for example at the end of the process of manufacturing the integrated circuit, by a so-called burning voltage.
U.S. Pat. No. 5,313,424 discloses an integrated memory having a self-repair function. A self-test unit tests the memory cells in the memory and then stores the address of faulty word lines in an appropriate address register. An activation signal with a high potential level is then supplied to the memory from outside, whereupon the severable electrical connection elements (fuses), which are a component part of a redundancy circuit, are destroyed to code the faulty word addresses stored in the address register. In this context, the fuses are destroyed by a high current that causes them to melt.
A further circuit configuration, disclosed in Published, Non-Prosecuted German Patent Application DE 198 43 470 A1, allows external analysis of the faults established. A memory unit for storing the address ascertained by the self-test unit has an output that is connected to a corresponding output of the circuit configuration for outputting the respectively stored address. Hence, if required, the manufacturer of the integrated circuit can establish whether faults have been established during the memory self-test and whether self-repair has been carried out. In addition, the manufacturer can establish the address of the faults established.
A volatile memory unit, such as an address register or address latch, can easily have information written to it by a self-test unit, since no voltages exceeding the normal signal level of the memory or large currents are required for this. The provision of a further, nonvolatile memory unit has the advantage that the memory self-test does not need to be repeated every time the volatile memory unit has lost the address stored in it (for example after the supply voltage has been turned off). If only one volatile memory unit were provided, the address of the normal units to be replaced would otherwise have to be established once again by the self-test (for example whenever the memory is initialized). The address is therefore permanently stored in the nonvolatile memory unit in a further step. To restore the memory content of the volatile memory unit, for example whenever the memory is initialized after the supply voltage is applied, the address stored in the nonvolatile memory unit then needs to be transferred to the volatile memory unit.
Particularly when electrical fuses are used, the burning procedure carried out to program the fuses does not always take place reliably. The transfer procedure carried out to restore the memory content of the volatile memory unit can therefore not always be completed reliably. The result of this is, by way of example, an incorrectly stored address in the volatile memory unit after the transfer procedure. It is therefore desirable to check whether the respective electrical fuse has been set such that the volatile memory element can reconstruct the originally stored information again from the state of the relevant fuse.
In the Published, Non-Prosecuted German Patent Application DE 198 43 470 A1, the output of the first volatile memory unit therein is a serial output for outputting the stored address bit by bit. The advantage of this is that the memory unit has only this one output. However, an associated serial shift procedure results in that the information stored in the volatile memory elements is first erased and, in the case of a shift register with feedback, the originally contained information is restored only after a complete cycle. By contrast, in the case of the present application, it is important for the information stored in the respective volatile memory element to be retained permanently in order to be able to compare it reliably at a later instant with the state reconstructed from the fuse.